Yttria is a relatively inert refractory material. A refractory material is any of various substances, such as ceramics, that are characterized by their suitability for use as structural materials at high temperatures. Refractory materials often are used for casting reactive or corrosive materials, including reactive metals. One example of such a reactive metal is titanium. Titanium reacts readily with oxygen to form oxygen-enriched titanium, which reduces the quality of articles cast from titanium. Titanium normally reacts with materials used to form the mold, such as oxides, thereby releasing oxygen and forming oxygen-enriched titanium.
Attempts to make a pH-stable, non-toxic yttria/colloidal-silica slurry have been unsuccessful ("slurry" is defined as a liquid dispersion of particles). Hence, such slurries are commercially impractical. For instance, Lassow's U.S. Pat. No. 4,703,806 states that a mold facecoat comprising yttria powder and aqueous colloidal silica produces a slurry which gels prematurely. Facecoats made from such slurries tend to crack during firing.
Colloidal silica is a widely used binder for casting molds. The colloidal silica (SiO.sub.2) usually used on a commercial basis have a silica content of approximately 30 percent, and are stabilized by an alkali (usually sodium oxide). Colloidal silicas are widely used as casting mold binders because the silicas are relatively inexpensive, stable, possess excellent room-temperature bonding characteristics, are not flammable, and do not require the use of organic solvents.
Such colloidal silicas have an optimum pH of less than ten, particularly about 9-10. A higher pH is unfavorable for maintaining SiO.sub.2 in a colloidal suspension. If the pH of a colloidal slurry made using SiO.sub.2 is greater than about 9-10, then the silica particles aggregate, and the lifetime of the slurry substantially decreases. Another consequence of silica particle aggregation is increased slurry viscosity. Increased slurry viscosity decreases the ability of the slurry to flow and drain from the mold after it is immersed into the slurry. The result is an irregular distribution of slurry on the surface of the mold. For these reasons, the viscosity of the slurry must be maintained within a certain acceptable range. Maintaining a proper pH is important for maintaining an appropriate slurry viscosity.
Horton's U.S. Pat. No. 4,947,927 (Horton) addresses some of the problems associated with yttria slurries. Horton states that an aqueous yttria slurry having a colloidal silica binder and a source of hydroxyl ion does not gel prematurely, as long as the slurry has a pH of at least 10.2, and preferably about 11.0. Horton specifically states that a slurry having an SiO.sub.2 /NaO.sub.2 equivalent dry weight ratio of 30-to-1 had a pH of less than 10.2 after six days experienced premature gelation to the extent that it settled and could not be redispersed. Due to their premature gelation tendencies, these slurries with a pH of less than 10.2 were all unsatisfactory for use in forming molds and/or cores. Horton, column 10, lines 48-54.
Horton also teaches using an organic base, in combination with the remaining ingredients of the yttria slurry, as the source of hydroxyl ion. Hydroxyl ions apparently are necessary to stabilize the yttria slurry in Horton's method. It is the pH of the solution, as well as the nature of the source of hydroxyl ions, that is important. With inorganic bases, the rate of silica dissolution is unacceptably high. Horton specifically teaches using an organic base, such as tetraethylammonium hydroxide, as the source of hydroxyl ions. A bulky organic base, such as tetraethylammonium hydroxide, helps prevent the dissolution of silica as the pH increases in Horton's patent.
Although yttria slurries have been recognized as being desirable for forming molds useful for casting reactive metals, yttria-zirconia slurries have not been considered for such slurries. Zirconia is more reactive with molten titanium than yttria. Compositions having about 95-percent zirconia are commercially available wherein the remaining 5 percent of the composition comprises yttria, alumina and other materials. Although such mixtures of zirconia and yttria are commercially available, they previously have not been used in the refractory industry.
In summary, the prior art teaches that slurries of yttria and colloidal silica are impractical, if not impossible, to produce. Horton teaches that yttria slurries may be produced if the pH of the slurry is maintained at a pH of greater than about 10.2. Moreover, a slurry at a pH of 11 is toxic, and therefore presents an environmental and health hazard. Finally, most commercially available colloidal-silica solutions, which typically are used in combination with other ingredients to form yttria slurries, have a pH in the range of about 9-10. Thus, it is preferable if the pH of a stable yttria slurry also was about 9-10.